Previous generations of sensors for chemical or biological compounds based on porous silicon photonic crystals possess surface functionality that only weakly interacts with non-polar organic compounds. For example, freshly prepared porous silicon possesses a silicon hydride terminated surface; however, this surface is not environmentally stable and it does not provide strong adsorption sites for many organic molecules. Oxidation of the silicon matrix yields hydrophilic surfaces with strong adsorption properties for alcohols and water, but not for non-polar organic compounds. These adsorption characteristics place a fundamental limit on the sensitivity of the device. Previously reported chemical modification reactions for porous silicon surfaces suffer from incomplete coverage and poor thermal or environmental stability.
Previous methods of templated nanowire and nanotube formation have used anodized aluminum oxide and etched polymer templates. See, Martin, C. “Nanomaterials: A Membrane-Based Synthetic Approach,” Science, Vol. 266, no. 5193, pp. 1961-66 (1994). The membranes in Martin's work contained cylindrical pores of uniform diameter, and were used to prepare polymers, metals, semiconductors, and other materials on a nanoscopic scale. This initial work was extended by others to produce nanowires and nanotubes of metals, metal oxides, conjugated polymers, inorganic salts, proteins, and metal chalcogen semiconductors. See, e.g., Nishizawa, M.; Menon, V. P.; Martin, C. R. “Metal Nanotubule Membranes with Electrochemically Switchable Ion-Transport Selectivity,” Science, 268, 700-702 (1995); F. Zhang, Y. Mao, T.-J. Park and S. S. Wong, “Green Synthesis and Property of Single-crystalline Alkaline Earth Metal Fluoride Nanowires,” Adv. Mater. 18, 1895-1899 (2006).